Catalytic reforming, or hydroforming, is a well established industrial process employed by the petroleum industry for improving the octane quality of naphthas or straight run gasolines. In reforming, a multi-functional catalyst is employed which contains a metal hydrogenation-dehydrogenation (hydrogen transfer) component, or components, substantially atomically dispersed upon the surface of a porous, inorganic oxide support, notably alumina. Noble metal catalysts, notably of the platinum type, are currently employed, reforming being defined as the total effect of the molecular changes, or hydrocarbon reactions, produced by dehydrogenation of cyclohexanes and dehydroisomerization of alkylcyclopentanes to yield aromatics; dehydrogenation of paraffins to yield olefins; dehydrocyclization of paraffins and olefins to yield aromatics; isomerization of n-paraffins; isomerization of alkylcycloparaffins to yield cyclohexanes; isomerization of substituted aromatics; and hydrocracking of paraffins which produces gas, and inevitably coke, the latter being deposited on the catalyst.
Platinum has been widely commercially used in recent years in the production of reforming catalysts, and platinum-on-alumina catalysts have been commercially employed in refineries for the last few decades. In the last decade, additional metallic components have been added to platinum as promoters to further improve the activity or selectivity, or both, of the basic platinum catalyst, e.g., iridium, rhenium, tin, and the like. Platinum-rhenium catalysts, by way of example, possess admirable selectively as contrasted with platinum catalysts, selectivity being defined as the ability of the catalyst to produce high yields of C.sub.5.sup.+ liquid products with concurrent low production of normally gaseous hydrocarbons, i.e., methane and other gaseous hydrocarbons, and coke.
Reforming reactions are both endothermic and exothermic, the former predominating, particularly in the early stages of reforming with the latter predominating in the latter stages of reforming. In view thereof, it has become the practice to employ a plurality of adiabatic fixed-bed reactors in series with provision for interstage heating of the feed to each of the several reactors. There are two major types of reforming. In semi-regenerative reforming, the entire unit is operated by gradually and progressively increasing the temperature to compensate for deactivation of the catalyst caused by the coke deposition, until finally the entire unit is shut down for regeneration, and reactivation, of the catalyst. In cyclic reforming, the reactors are individually isolated, or in effect swung out of line by various piping arrangements, the catalyst is regenerated to remove the coke deposits, and then reactiviated while the other reactors of the series remain on stream. A "swing reactor" temporarily replaces a reactor which is removed from the series for regeneration and reactivation of the catalyst, and is then put back in series. In either type of reforming, hydrogen is produced in net yield, the product being separated into a C.sub.5.sup.+ liquid product, e.g., a C.sub.5 /430.degree. F. product, and a hydrogen rich gas a portion of which is recycled to the several reactors of the process unit. In most refineries, hydrogen from the reforming operation is also required for operation of a Hydrofiner; proper operation of the Hydrofiner being essential for removing sulfur from the feed to the reforming unit.
Essentially all petroleum naphtha feeds contain sulfur, a well known catalyst poison which can gradually accumulate upon and poison the catalyst. Most of the sulfur, because of this adverse effect, is generally removed from feed naphthas, particularly by Hydrofining, or hydrogen treating. In use of the more recently developed multi-metallic platinum catalysts wherein an additional metal, or metals, hydrogenation-dehydrogenation component is added as a promoter to the platinum, it has become essential to reduce the feed sulfur to only a few parts, per million parts by weight of feed (ppm). For example, in the use of platinum-rhenium catalysts it is generally necessary to reduce the sulfur concentration of the feed well below about 10 ppm, and preferably well below about 2 ppm, to avoid excessive loss of catalyst activity and C.sub.5.sup.+ liquid yield. The role of sulfur on the catalyst presents somewhat of an anomaly because the presence of sulfur in the feed can adversely affect the activity of the catalyst and reduce liquid yield; and yet, sulfiding of the polymetallic catalyst species, which is a part of the catalyst reactivation procedure, has been found essential to suppress excessive hydrogenolysis which is particularly manifest when a reactor is first put on stream after regeneration and reactivation of the catalyst. Excessive hydrogenolysis caused by use of these highly active catalysts can not only produce acute losses in C.sub.5.sup.+ liquid yield through increased gas production, but the severe exotherms which accompany operation in a hydrogenolysis mode can seriously damage the catalyst, reactor, and auxiliary equipment.
In semi-regenerative reforming, for example, it has been found that when the reactors of a unit which contain fresh, or regenerated, reactivated highly active rhenium promoted platinum catalysts are put back on-stream, the start-up period is characterized by an initial loss of catalyst activity and loss of C.sub.5.sup.+ liquid yield. The same phenomenon is observed in cyclic reforming. When a platinum-rhenium catalyst loaded reactor is reinserted in the multiple reactor series of the unit, albeit it contains a fresh catalyst, or a regenerated, reactivated, sulfided catalyst, there occurs an initial upset period when the catalyst activity and C.sub.5.sup.+ liquid yield of the unit is reduced. It has been observed that this effect is first noted in the reactor immediately downstream of the swing reactor which when first put on-stream contains a freshly sulfided catalyst. A quantity of sulfur is released when the freshly sulfided catalyst is contacted with the feed, the sulfur wave travelling downstream from one reactor to the next of the series. Concurrent with the sulfur wave there results a loss in C.sub.5.sup.+ liquid yield which, like a wave, also progresses in seratim from one reactor of the series to the next until finally the C.sub.5.sup.+ liquid yield loss is observed throughout the unit. Over a sufficiently long period after the initial decline in C.sub.5.sup.+ liquid yield loss, the C.sub.5.sup.+ liquid yield in the several reactors of the unit, and consequently the overall performance of the unit, gradually improves, though often the improvement is not sufficient to return each of the reactors of the unit, or unit as a whole, to its original higher performance level.
It is desirable, because of this phenomenon, to avoid the use of high sulfur feeds, particularly during start-up. Quite obviously, however, this is not always possible; and all too often it is simply infeasible to use a sweet feed. It is often necessary to put the unit on-stream with a high sulfur feed (e.g., one which contains 25-200 ppm sulfur) until sufficient hydrogen is generated to operate the naphtha Hydrofiner. In employing polymetallic catalysts, e.g., platinum-rhenium catalysts, this presents a significant problem, since high hydrogen sulfide following oil-in results in severe catalyst activity and C.sub.5.sup.+ liquid yield losses. The consequences of this adverse start-up have long range effects, and the losses occasioned on start-up are only partially reversed even after the feed sulfur has dropped to an acceptable concentration (&lt;2 ppm) after the Hydrofiner is operating satisfactorily. Coke formed early in the run on a highly active catalyst in a high sulfur environment is particularly detrimental. Purging hydrogen sulfide from the system is a slow process, even with recycle gas driers having good sulfur capacity.
It is, accordingly, the primary object of this invention to provide a new and improved process which will obviate these and other disadvantages of the present start-up procedures for reforming units, particularly those employing highly active polymetallics, or promoted noble metal containing catalysts.
A specific object is to provide a new and novel operating procedure for reforming units, notably one which will effectively suppress sulfur release and the normally expected initial period of C.sub.5.sup.+ liquid yield decline which occurs with metal promoted platinum catalysts, particularly rhenium promoted platinum catalysts.
These objects and others are achieved in accordance with the present invention embodying an improved process wherein, on start-up of a reforming unit, a sulfur-containing naphtha feed is fed at reforming conditions over a platinum-catalyst containing lead reactor of a series while bypassing subsequent reactors of the series, the product therefrom is separated into hydrogen-containing gas and C.sub.5.sup.+ liquid fractions, the hydrogen-containing gas fraction is desulfurized and dried and recycled to the platinum-catalyst containing lead reactor and, after sufficient hydrogen has been generated for operation of the Hydrofiner, product from the platinum-catalyst containing lead reactor is fed to subsequent reactors of the series which contain more sulfur-sensitive catalysts, notably polymetallic, or metal promoted platinum catalysts.
In other words, in accordance with this invention, during start-up with a relatively high sulfur feed, only the platinum catalyst of the lead reactor is contacted with high sulfur feed, and succeeding reactors to which the product of the lead reactor is normally fed are bypassed. Since the platinum catalyst is much less sulfur sensitive than metal promoted platinum catalysts, the lead reactor catalytically reforms the feed to produce an admixture of hydrogen-containing gases and C.sub.5.sup.+ liquids. A hydrogen enriched gas is separated from the admixture and passed, in whole or in part, to the Hydrofiner. Hydrogen is used in the Hydrofiner to desulfurize the sulfur-containing feed to a preselected concentration, depending on the requirements of the reforming unit. The concentration of sulfur is reduced in the Hydrofiner feed, with the consequence that the sulfur concentration in the feed to the lead reactor of the reforming unit is also reduced, sulfur being purged from the platinum catalyst. When the hydrogen sulfide concentration in the output gas product from the lead reactor is reduced below about 10 parts per million by volume, vppm, preferably below about 5 vppm, then the downstream reactors of the unit are brought on stream at reforming conditions to produce reformate of the desired octane. The C.sub.5.sup.+ liquid originally reformed in the lead reactor, if desired, can be further reformed in the series of reactors to further improve the octane, or blended in with the total product.
These features and others will be better understood by reference to the following more detailed description of the invention, and to the drawing to which reference is made.